The demodulation of FM-signals with the assistance of quadrature demodulators is known. Briefly, this usually involves having an input signal fed at one time directly to the input of a multiplier and at the same time through a phase shifter circuit (LC-circuit). The phase difference of the signals generated therewith results in a proportional output voltage. This voltage corresponds to the modulated signal as compared to the carrier signal. Accordingly, a considerable alignment of the signals is necessary to obtain a minimum of distortion and to have the zero-axis-crossing of the output voltage in center frequencies.
In this regard, the use of a phase locked frequency analysis circuit utilizing phase control circuit or phase locked loop (PLL) has advantages. An FM-demodulator incorporating the PLL circuit usually consists of a multiplier, low pass filter and a voltage controlled oscillator (VCO). The multiplier and the low pass filter form a phase detector whose output voltage is a voltage corresponding to the momentary phase difference between oscillator signal and the input or modulated signal. With this voltage, the momentary phase of the oscillator signal is readjusted to the momentary phase of the input signal. When the momentary phases of both signals are synchronized, the loop is "locked".
When a linear condition exists between the control voltage, which is equal to the output voltage of the multiplier, and the frequency of the VCO, the control voltage becomes an image of the input signal and represents the demodulated signal. However the "VCO" has a linear voltage-frequency-characteristic-curve and it is difficult to obtain a frequency range in an intermediary frequency (for example, at 10. MHz) in a simple manner without linearization members.